High power semiconductor rectifiers are key components in power electronic systems, such as high-voltage direct current (HVDC) electric power transmission systems, control electronics, power supplies and motor drives. A higher electric breakdown field strength, a higher thermal conductivity, a lower intrinsic carrier concentration and a higher saturated drift velocity compared to silicon make silicon carbide (SiC) a favorable material for high-power devices. Silicon carbide based rectifiers can be made with a significantly thinner drift layer compared to silicon based rectifiers having the same blocking voltage due to the higher electric breakdown field strength of silicon carbide compared to that of silicon. Known semiconductor rectifiers include P-i-N diodes, Schottky diodes and Junction Barrier Schottky (JBS) diodes. The Schottky diode is a unipolar diode, in which the current conduction is governed by majority carriers (electrons). It offers a relatively high switching speed but has a higher leakage current compared to a P-i-N diode constructed from the same semiconductor material. The lower switching speed of the P-i-N diode compared to that of a Schottky diode made from the same semiconductor material is due to a higher reverse recovery charge resulting from minority (holes) and majority (electrons) carriers being involved in the current conduction. The JBS diode can combine a low threshold voltage and high switching speed similar to that of a Schottky diode with good blocking characteristics similar to that of a P-i-N diode. It has the structure of a Schottky diode with a p-n junction grid integrated into its drift region.
Silicon Schottky diodes are not used as high-power rectifiers because of their low blocking capability. SiC Schottky diodes have a much lower reverse leakage current and a higher reverse (or blocking) voltage than silicon Schottky diodes. Compared to a SiC P-i-N diode, a SiC Schottky diode has the advantage of a lower threshold voltage and lower switching losses due to a lower reverse recovery charge.
SiC Schottky diodes are commercially available since 2001, challenging established silicon P-i-N diodes in applications from 300V to 3.3 kV. Depending on the application, SiC rectifiers may be required to handle various levels of surge currents, sometimes as high as 10 times the value of the nominal current rating. In case of such high surge currents the SiC Schottky diode may be destroyed by the heat generation due to thermal losses in the Schottky junction.
A SiC semiconductor rectifier with improved blocking characteristics and surge current capability compared to the SiC Schottky diode is a SiC JBS diode. In FIG. 1 a partial cross section of the known SiC JBS diode 10 is shown. It comprises a semiconductor wafer having a first main side 12 and a second main side 13. In an orthogonal projection onto a plane parallel to the first main side 12, the semiconductor wafer has an active region AR surrounded by an edge termination region TR. In the order from the second main side 13 to the first main side 12, the semiconductor wafer includes an n+-type cathode layer 14 and an n−-type drift layer 15. Adjacent to the first main side p+-type emitter layer portions 16 form the p-n junction grid pattern, a p+-type transition region 17 is formed in the active region AR along the boundary between the active region AR and the edge termination region TR. In the edge termination region TR a p-type junction termination extension (JTE) region 18 is formed adjacent to the first main side 12. On the first main side 12 a passivation layer 19 is formed in the edge termination region TR and overlapping with the transition region 17 in the orthogonal projection onto the plane parallel to the first main side 12. In the active area AR the first main side 12 is covered by metal electrode layer 21 forming a Schottky contact with the n−-type drift layer 15 and forming an ohmic contact with the p+-type emitter layer portions 16 and with the p+-type transition region 17. A top metal 22 is formed on the metal electrode layer 21. The metal electrode layer 21 and the top metal 22 overlap with and extend onto the passivation layer 19. On the second main side 13 there is formed a backside metallization 23 as an anode electrode. The electrode layer 21 is exemplarily made of titanium (Ti), nickel (Ni), tungsten (W), cobalt (Co) or a combination thereof. The top metal 22 is exemplarily made of aluminum (Al).
In operation the improved blocking characteristics of the SiC JBS diode is due to the fact that the depletion region, which extends from the p-n junction grid under reverse bias conditions can protect the Schottky contact from a high electric field, which can eventually result in premature breakdown due to increased leakage currents. The depletion layer extending from each pair of two neighbouring stripe-shaped p+-type emitter layer portions 16 can pinch off the Schottky contact between these two neighbouring stripe-shaped p+-type emitter layer portions 16.
In the JBS diode as shown in FIG. 1 the Schottky contacts have a lower threshold voltage than the p-n junctions. Therefore, during normal operation (flow of a current equal to or below the nominal current), under a relatively low forward bias, the current conduction is governed by the unipolar current flow through the Schottky contacts. In case of a surge current a higher voltage drop will develop over the device and the p-n junction grid switches into on-state so that bipolar current can flow through the p-n junction grid. The electrical resistance of the p-n junction is lower than that of the Schottky junction when holes are injected through the p-n junction into the drift layer. Accordingly, the p-n junction grid allows the flow of higher surge currents than SiC Schottky diode without a p-n junction grid.
From U.S. Pat. No. 8,232,558 B2 there is known a JBS diode comprising a p-type surge current protection region formed in an n−-type drift layer in addition to stripe-shaped p-type JBS regions. The surge current protection region has a higher lateral width than the p-type JBS regions to increase the surge current capability. In U.S. Pat. No. 8,232,558 B2 there is not described any p-type transition region along the boundary to a termination region. Also, the p-type stripe-shaped emitter regions are separated from the p-type surge current protection region by the drift layer. The problem in the JBS diode disclosed in this prior art document is that a surge current may first (i.e. in an initial phase of a surge current) be concentrated in the protection region and result in local overheating of the device.
From the article “Current distribution in the various functional areas of a 600V SiC MPS diode in forward operation” by Rupp et al. in Materials Science Forum Vols. 717-720 (2012) pp 929-932 it is known to arrange the p-n junction grid in form of a grid having a hexagonal cell pattern. Some cells consist of a large p-area without Schottky part to allow minority carrier injection without snapback phenomenon in the IV characteristic (negative differential resistance). The described JBS diode also includes a large p+-type transmission region in the edge termination region. It is described that this large p+-type transmission region in the edge termination region contribute first by minority carrier injection, whereas the smaller p+-type hexagonal cells and the p+-type grid follow only subsequently with increasing current density.